Organic scintillating layer in a photographic element



United States Patent 3,491,235 ORGANIC SCINTILLATING LAYER IN A PHOTOGRAPHIC ELEMENT Allan G. Millikan, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Aug. 13, 1964, Ser. No. 389,457 Int. Cl. H01j 37/26, 37/22 U.S. Cl. 250-495 15 Claims ABSTRACT OF THE DISCLOSURE Recording elements, e.g., films, for electron beam recording comprise fluorescent layer, electrically conductive layer and recording layer on a support. The fluorescent layer comprises organic fluorescent material dispersed in a hydrophilic colloid (e.g., gelatin). This permeable fluorescent layer over a silver halide recording layer admits processing liquids through to the silver halide. Fluorescent layer is made by dispersing organic solution of fluorescent compounds in aqueous colloid solution, coating and drying. Method is also described for making a fluorescent photographic image using gelatin silver halide emulsion containing fluorescent material.

This invention relates to improved electron-sensitive recording elements, more particularly to electron-sensitive films and plates having fluorescent layers incorporated therein, and to methods and materials for making such elements, and to methods for using such elements.

Electron-sensitive films and plates are useful in several known applications for recording an image by direct exposure to an electron beam, for example in electron microscopes, data recording apparatus, etc.

Direct electron recording films and plates comprise a layer of electron-sensitive material that forms an image in the layer by direct imagewise exposure in an electron beam. The image produced may be directly visible as in a layer of monomeric material that polymerizes upon electron beam exposure or a layer of photographic printout emulsion, or it may be a latent image that requires further development to produce a visible image as in a gelatin-silver halide emulsion. In preferred embodiments of the invention we employ a gelatin-silver halide emulsion as the sensitive layer.

In some recording apparatus that employs direct electron beam recording films, an image recorded on a film or plate is read back or viewed by projecting a beam of ultra-violet or other radiation through the developed film onto a fluorescent screen to produce a fluorescent image on the screen. For some uses, however, it is preferable to view the image directly on the film by irradiating a fluorescent layer coated on the film or plate. See B. Miller, Electron Beam Read-Out Shows Potential, Aviation Week & Space Technology, Apr. 13, 1964, pp. 107-110.

An object of the invention is to provide a method for coating directly onto a sensitive film or plate a fluorescent layer that comprises a dispersion of colloidal particles of a fluorescent hydrophobic organic compound in a hydrophilic film-forming binder. Other objects are to provide an electron-sensitive film useful for direct electron recording and having thereon a thin fluorescent layer of the kind just described and to provide methods for making and using such film.

In a preferred embodiment of the present invention a fluorescent layer on an electron recording element comprises a continuous film of a water-permeable filmforming binder and, dispersed in this layer, colloidal particles of a water-insoluble organic fluorescent compound. In some embodiments the fluorescent layer may also be the light-sensitive layer, e.g. a gelatin-silver halide emulsion in which colloidal particles of fluorescent material are dispersed. In other embodiments fluorescent material may be dispersed in a separate layer coated on the same side of the support either over or under the imageforming or image-carrying layer, and in some embodiments the fluorescent layer may be on the side of the film support opposite the image-forming or image-carrying layer. In some preferred embodiments we also provide a conducting layer having electrical surface resistivity less than 10 ohms per square to prevent accumulation of static electricity during exposure in an electron beam.

One method for preparing a colloidal dispersion of fluorescent organic compound in the film-forming binder is to ball-mill an aqueous solution of the film-forming binder with particles of the organic fluorescent compound until a colloidal dispersion is formed, and then coating the emulsion on a film support and evaporating water to dry the coated emulsion.

Compositions for making coated fluorescent layers according to the invention may be prepared by first dissolving at least one substantially water-insoluble fluorescent organic compound in a volatile, substantially water-insoluble organic solvent. This water-immiscible solution is then stirred into an aqueous solution of gelatin or other hydrophilic film-forming binder material, and the mixture is mechanically homogenized, as can be conveniently done in a colloid mill, to form a colloidal dispersion of the hydrophobic solution in a continuous phase of the aqueous medium. This homogenized suspension is coated on a suitable support, such as a photographic film base or a glass plate, and dried to evaporate all or at least the greater part of the organic solvent and water, leaving a continuous film of the binder in which colloidal particles of the organic fluorescent material are dispersed. Alter natively, the emulsion may be dried by evaporating water and solvent from the aqueous suspension before coating, after which the dry residue is redispersed in water and this redispersion is coated onto an electron recording element and dried. The redispersion may also be mixed with a silver halide emulsion before coating to make a fluorescent emulsion for coating.

The preferred image-forming layer for an electronsensitive element is a gelatin-silver halide emulsion layer, which may be an ordinary photographic emulsion, preferably a fine-grain, high-resolution type. Gelatin is the preferred hydrophilic film-forming binder for both the silver halide layer and the fluorescent layer, though this invention includes elements in which other hydrophilic film-forming colloids may be used, for example aqueous solutions of polyvinyl alcohol and the like.

A variety of fluorescent organic compounds are known that are substantially water-insoluble, and for each there will be a variety of water-immiscible organic solvents that can be used for making coating compositions according to the invention. The hydrophobic organic fluorescent compounds and the hydrophobic solvents listed in Table 1 are only preferred examples that can be used according to the invention and the invention is not limited to these water-insoluble organic compounds that fluoresce in either electromagnetic or electron-beam irradiation, or in both, may be used in accordance with the invention. In electron-beam recording films which are to be read back by irradiation directly in an electron beam, we prefer to use a fluorescent organic compound having a decay rate in the magnitude of 10" seconds, or faster. Suitable organic fluorescent compounds are those referred to as organic fluors and organic scintillators in Organic Scintillation Detectors by E. Schram and R. Lombaert, Elsevier Publishing Co., 1963. Some organic fluorescent compounds will be more suitable than others for a particular use and examples of some that we have found especially suitable for our use are the first two listed in Table 1, i.e. (TPB and POPOP). A mixture of these two compounds is especially preferred.

TABLE 1 Organic fluorescent Fluorescence observed compound Solvent in electron beam 1,1,4,4-tetraphenyl- Cyelohexanone Deep blue at 20 kev.

butadiene (referred to as T B). 1,4-bis-2-(5-phenyloxado Bright blue. zolyl)-benzene (referred to as POPOP). p-TerphenyL- Benzene Good deep blue. Anthracene... Cyclohexanone- Goztgdkpurple blue at p-Quarterphenyl Benzene Purple. Leucophor B (triazinyl Cyclohexanone D0.

amino stilbene).

EXAMPLE I To demonstrate coating procedures and fluorescent properties of fluorescent layers according to the invention, several coating compositions were prepared, each containing a different one of the organic fluorescent com pounds listed in Table 1. In ml. of the solvent shown in Table 1 for each respective fluorescent compound, was dissolved 0.3 grams of that compound. The solution was then added with stirring to 50 ml. of a 10 percent gelatin solution at 40 C., with one ml. of a percent saponin solution added as a dispersing agent. Each mixture was then passed through a colloid mill several times to homogenize the mixture, dispersing the hydrophobic solution as a colloidal dispersion in the continuous aqueous phase. The homogenized emulsions were then coated on a flexible photographic film support at a thickness to give about 100 mg. of fluorescent compound per square foot. The coatings were then dried by evaporating solvent and water from the coating in vacuum to produce a continuous gelatin film which contained the organic fluorescent material evenly dispersed therein as colloidal particles. The films were tested for fluorescence under electron bombardment (6 to 26 kev.) and the fluorescence of the respective layers while in the electron beam was observed. Results are listed in Table 1.

EXAMPLE II To demonstrate a different coating procedure and a preferred coating composition a coating composition containing the first two compounds listed in Table 1 (TPB & POPOP) was prepared as follows. One gram of TPB and 0.1 gram of POPOP were dissolved in 40 ml. cyclohexanone. The hydrophobic solution was poured into 150 grams of 10 percent gelatin solution containing 7.0 ml. of 13 percent Alkanol B aqueous solution as a dispersing agent. This mixture was milled five times in a colloid mill to thoroughly disperse the hydrophobic solution in the gelatin solution. This emulsion was chill-set, noodled and dried to remove practically all of the water and organic solvent. After this, the composition was redispersed in water and the fluorescent compounds were observed to be dispersed in the aqueous medium as colloidal particles, essentially non-crystalline and so small as to be almost unresolvable by a microscope. The redispersed emulsion was coated on a support film of 0.07-inch thick polyethylene terephthalate and then dried by evaporating most of the water. This coating fluoresced well in a 15 kev. electron beam. The presence of POPOP reduces crys tallization of the colloidal particles and shifts the wavelength of the fluorescence farther into the visible spectrum.

EXAMPLE III A portion of the redispersed emulsion of Example H was coated over a sensitive layer of silver halide gelatin emulsion of the Lippman type on a polyethylene terephthalate film support. The film was exposed in an electron beam to form a latent image in the sensitive layer and a silver image was developed and fixed by conventional photographic processing using a Kodak D-l9 developer solution and hypo fixer. The processing solutions easily permeated the fluorescent layer to reach the silver halide layer. After processing of the film, the fluorescent layer remained firmly secured to the film and exhibited a good fluorescent image when irradiated in an electron beam. The mass of the fluorescent layer over the sensitive layer was found to reduce speed of the silver halide emulsion layer to electron exposure to some extent and so, in order to reduce the mass of material in the over-coated layer, another coating emulsion was prepared as before except with concentration of fluorescent compounds in the coating composition doubled, using the same quantities of cyclohexanone, gelatin and dispersing agent. This coating solution could be coated as a thinner layer to give the same concentration per square foot of fluorescent material. Films coated with this more concentrated composition were found to have a faster response to electron exposure.

EXAMPLE IV An electron-sensitive element with fluorescent material dispersed in the image-forming layer was prepared as follows. A very fine grain silver bromide gelatin emulsion of the Lippman type was prepared. To 61 grams of this emulsion, containing about mole of silver halide, there was added 8.5 ml. of a dispersion prepared as follows: 1.0 gram TPB, and 0.1 gram POPOP were dissolved in 20 ml. cyclohexanone and added to 75 ml. 10 percent gelatin solution which contained 40 ml. of 13 percent Alkanol B and the mixture was milled to produce a colloidal dispersion of the hydrophobic solution in the aqueous gelatin phase. This dispersion was thoroughly mixed into the silver halide emulsion and the mixed emulsion was coated onto a glass plate at a coverage of about 618 mg. of silver and mg. of fluorescent compound per square foot, and dried. A plate so coated was first exposed in an electron beam to produce a line image and then processed in Kodak D-l9 developer for 1 minute, fixed, washed, and dried. This plate was then irradiated on the coated side in an ultraviolet beam and bright fluorescence was observed in the background areas which contained no silver, while in image areas containing silver there was considerably less fluorescence, resulting in a positive fluorescent image. A second plate prepared as before was exposed imagewise in an electron beam to produce a line image, and then processed in Kodak tanning developer for 1 minute, fixed, and then washed in 110 F. running water to remove untanned gelatin in the background areas leaving a high contrast relief image containing silver and fluorescent compound. The plate was dried and then viewed in ultraviolet radiation. A negative fluorescent image was observed.

The plate carrying the relief image described above was processed in Kodachrome K-12 bleach, washed and fixed in hypo to remove the silver, leaving a negative relief gelatin image containing only the fluorescent compound. This relief image gave a much higher fluorescence than the unbleached image when irradiated in ultraviolet or in an electron beam.

EXAMPLE V An electron-sensitive film was prepared as in Example III except the fluorescent layer was coated on the film support under the Lippman type photographic emulsion.

EXAMPLE v1 An electron-sensitive film was prepared as in Example III except the fluorescent layer was coated on the side of the film support opposite the photographic emulsion layer.

EXAMPLE VII Preparations of finely divided organic sciniillators may be prepared by ball-milling techniques. This technique has the advantage over the previously described methods, that higher scintillator to binder ratios can be made. A typical ball-milled preparation may be prepared as follows: g. of TPB and one gram of POPOP were added with 5 g. of 10 percent aqueous solution of Alkanol B as a dispersing agent to 100 g. distilled water. This mixture was ball-milled for two hours and then g. of a 10 percent aqueous gelatin solution was added and the mixture was ball-milled for an additional 64 hours. The mixture was then eluted and 90 g. of a 10% gelatin solution added with stirring at 40 C. The dispersion so prepared was then coated on polyethylene terephthalate film supports at various thickness to give respectively, 100 milligrams per square foot of TPB, 50 milligrams per square foot of TPB, milligrams per square foot of TPB and 10 milligrams per square foot of TPB. All of these coatings showed excellent fluorescence in ultraviolet radiation and in radiation by electron beam. Fluorescent coatings prepared by ball-milling may be substituted for the fluorescent coating described in previous examples in making photographic and electron-sensitive films.

On electron-sensitive recording elements used in electron beam recording it is advantageous to have a layer of electrically conductive material having surface resistivity less than 10 ohms per square, coated on the same side of the support that carries the sensitive layer. This conductive layer serves to prevent accumulation o fa static charge and consequent image distortion where an electron beam strikes the sensitive element. This conductive layer may be the outermost layer on the emulsion side of the support. However, in some embodiments, it can be effectively located beneath other coated layers on the support, i.e. beneath the fluorescent layer, the sensitive layer, or both. If the conductive layer is beneath permanent layers, then of course it must be able to withstand whatever processing chemicals are to be used. If the layer is outermost it must be substantially electron transparent and it must be permeable by whatever processing solutions are used. Preferably it is one that will not be removed by processing chemicals and is optically transparent. This permanent conductive outer layer will prevent static accumulations on the surface during imagewise exposure in an electron beam and again when the finished film is irradiated in an electron beam to view a fluorescent image. The outer conductive layer may also be one that is removed during processing, preferably by the same chemicals used for photographic development, and in this case it need not be visually transparent though it must be substantially electron transparent.

A preferred conductive layer for use in our fluorescent, electron-sensitive films consists of a layer of film-forming resin binder in which are dispersed colloidal particles of a semi-conducting metal compound such as cuprous iodide or silver iodide, either as a colloidal dispersion or as a complexed solute. Conducting layers of this type are described in the copending US. patent application of Donald Trevoy, Ser. No. 56,648, filed Sept. 19, 1960.

A direct-electron recording element comprising a conducting layer and a fluorescent layer according to the present invention is illustrated in the following example.

EXAMPLE VIII Cuprous iodide (2.4 g.) was dissolved in a mixture of 200 ml. methyl ethyl ketone and 4.0 ml. of trimethyl phosphite, then 40 ml. of a 5% solution of a terpolymer poly (methylacrylate-vinylidene chloride-itaconic acid) in 90% methyl ethyl ketone and 10% cyclohexanone was added. The solution was filtered and then machine coated by head application on a subbed polyester film support to give a coverage of 5 mg. of copper per square foot. The coating was dried at 110 C. and then cured at 120 C. for 10 minutes. The coating was clear and surface resistivity was 1.7 10 ohms per square. A protective layer of Vinylite VMCH was solution-coated from a ketone solvent over the conducting layer. This protective coating was dried at C. and cured at C. for four minutes. Over this protective layer a thin subbing of cellulose nitrate (from a 1.4% solution in methanol) was applied to improve adhesion. A gelatin subbing was applied and a gelatin-silver halide photographic emulsion of the Lippman type was coated over the subbing. A fluorescent coating was coated over the sensitive layer of silver halide gelatin emulsion as in Example III above. Presence of the conducting layer reduced distortion caused by electron accumulation on the film and for this reason a better image was obtained than with the element described in Example III.

It will be understood that modifications and variations may be made within the scope of the invention as described above and as defined in the following claims.

We claim:

1. An electron-beam-sensitive recording element comprising a film support and coated thereon (1) a discrete recording layer of electron-beam-sensitive undeveloped silver halide image-forming material positioned on said element to receive actinic radiation directed at said element and positioned to be chemically developed as part of said recording element without removal of any of the herein defined layers of said element; (2) a discrete fluorescent layer comprising organic fluorescent material having a decay rate in the magnitude of 10 seconds or faster, said fluorescent layer being adapted to fluoresce in actinic radiation directed upon said element and being positioned outward from said recording layer with respect to said support and being permeable by aqueous fluids and adapted for photographic processing of said sensitive layer Without removal of said fluorescent layer by such fluids; and (3) an electrically conductive layer having surface resistivity less than 10 ohms per square.

2. A recording element defined by claim 1 wherein said support is a transparent film support.

3. A recording element defined by claim 2 wherein said film support is a polyethylene terephthalate film.

4. A recording element defined by claim 1 wherein said silver halide image-forming material is a gelatin-silver halide photographic emulsion.

5. A recording element defined by claim 1 wherein said fluorescent layer consists of water-insoluble colloidal size particles of the defined organic fluorescent material dispersed in a hydrophilic colloid binder.

6. A recording element defined by claim 5 wherein the said hydrophilic colloid binder of said fluorescent layer is gelatin.

7. A recording element defined by claim 1 wherein said electrically conductive layer comprises colloidal particles of a semi-conducting metal compound dispersed in a filmforming resin binder.

8. A recording element defined by claim 7 wherein said semi-conducting metal compound consists of cuprous iodide.

9. An electron-beam-sensitive recording element comprising a transparent polyethylene terephthalate film sup port; a discrete recording layer of an undeveloped photographic silver halide gelatin emulsion positioned on said support to receive actinic electron-beam radiation directed at said element and positioned to be chemically developable as part of said recording element by aqueous processing solutions without removal of any of the herein defined layers of said element; a discrete fluorescent layer comprising water-insoluble colloidal size particles of organic fluorescent material having decay rate in the magnitude of 10* seconds or faster, dispersed in a hydrophilic colloid binder, said fluorescent layer being adapted to fluoresce in actinic radiation directed at said element and being positioned outward from said recording layer with respect to said support and being permeable by aqueous processing fluids without removal of the fluorescent layer by such fluids; and an electrically conductive layer having surface resistivity less than ohms per square.

10. An electron-beam recording element comprising a visually transparent support and coated on a single side thereof in combination:

(a) a visually transparent, electrically conductive layer having surface resistivity less than 10 ohms per square, and coated outward therefrom (b) a discrete, image-recording layer comprising undeveloped, radiation-sensitive silver halide, and coated outward therefrom (c) a discrete fluorescent layer that transmits electronbeam radiation sufliciently for image recording in said recording layer, the fluoresces in actinic radiation, and that is permeable by aqueous developer and fixer solutions for processing said silver halide without removal of any substantial part of said fluorescent layer.

11. An electron-beam-sensitive recording element defined by claim 10 wherein said fluorescent layer comprises water-insoluble colloidal size particles of fluorescent material having decay rate in the magnitude of 10- seconds or faster dispersed in a hydrophilic colloid binder.

12. An 'electron-beam-sensitive recording element defined by claim 11 wherein said fluorescent material comrecording layer, and comprising the improvement wherein (a) said recording layer is a discrete layer comprising undeveloped radiation sensitive silver halide and (b) said fluorescent layer is a discrete layer that is coated outward from said recording layer on the same side of said support and that transmits electron-beam radiation sufficiently for image recording in said recording layer and that fluoresces in actinic radiation and that is permeable by aqueous developer and fixer solutions for processing said silver halide.

14. An improved electron-beam recording element defined by claim 13 wherein said fluorescent layer comprises water-insoluble colloidal size particles of fluorescent material having decay rate in the magnitude of 10* second or faster dispersed in a hydrophilic colloid binder.

15. An improved electron-beam recording element defined by claim 14 wherein said fluorescent material comprises a water-insoluble organic fluorescent compound.

References Cited UNITED STATES PATENTS 1,448,456 3/1923 Leuy et al 9682 3,282,697 11/1966 Blank et al. 9682 2,801,171 7/1957 Fierke a a1 96109 X 3,303,341 2/1967 Fram et al. 9682 X FOREIGN PATENTS 163,903 6/1921 Great Britain.

NORMAN G. TORCHIN, Primary Examiner R. E. FICHTER, Assistant Examiner US. Cl. X.R. 

